src/runtime/internal/sys/intrinsics.go

// Copyright 2016 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// +build !386

package sys

// Using techniques from http://supertech.csail.mit.edu/papers/debruijn.pdf

const deBruijn64 = 0x0218a392cd3d5dbf

var deBruijnIdx64 = [64]byte{
	0, 1, 2, 7, 3, 13, 8, 19,
	4, 25, 14, 28, 9, 34, 20, 40,
	5, 17, 26, 38, 15, 46, 29, 48,
	10, 31, 35, 54, 21, 50, 41, 57,
	63, 6, 12, 18, 24, 27, 33, 39,
	16, 37, 45, 47, 30, 53, 49, 56,
	62, 11, 23, 32, 36, 44, 52, 55,
	61, 22, 43, 51, 60, 42, 59, 58,
}

const deBruijn32 = 0x04653adf

var deBruijnIdx32 = [32]byte{
	0, 1, 2, 6, 3, 11, 7, 16,
	4, 14, 12, 21, 8, 23, 17, 26,
	31, 5, 10, 15, 13, 20, 22, 25,
	30, 9, 19, 24, 29, 18, 28, 27,
}

// Ctz64 counts trailing (low-order) zeroes,
// and if all are zero, then 64.
func Ctz64(x uint64) uint64 {
	x &= -x                      // isolate low-order bit
	y := x * deBruijn64 >> 58    // extract part of deBruijn sequence
	y = uint64(deBruijnIdx64[y]) // convert to bit index
	z := (x - 1) >> 57 & 64      // adjustment if zero
	return y + z
}

// Ctz32 counts trailing (low-order) zeroes,
// and if all are zero, then 32.
func Ctz32(x uint32) uint32 {
	x &= -x                      // isolate low-order bit
	y := x * deBruijn32 >> 27    // extract part of deBruijn sequence
	y = uint32(deBruijnIdx32[y]) // convert to bit index
	z := (x - 1) >> 26 & 32      // adjustment if zero
	return y + z
}

// Bswap64 returns its input with byte order reversed
// 0x0102030405060708 -> 0x0807060504030201
func Bswap64(x uint64) uint64 {
	c8 := uint64(0x00ff00ff00ff00ff)
	a := x >> 8 & c8
	b := (x & c8) << 8
	x = a | b
	c16 := uint64(0x0000ffff0000ffff)
	a = x >> 16 & c16
	b = (x & c16) << 16
	x = a | b
	c32 := uint64(0x00000000ffffffff)
	a = x >> 32 & c32
	b = (x & c32) << 32
	x = a | b
	return x
}

// Bswap32 returns its input with byte order reversed
// 0x01020304 -> 0x04030201
func Bswap32(x uint32) uint32 {
	c8 := uint32(0x00ff00ff)
	a := x >> 8 & c8
	b := (x & c8) << 8
	x = a | b
	c16 := uint32(0x0000ffff)
	a = x >> 16 & c16
	b = (x & c16) << 16
	x = a | b
	return x
}